Closer

Will Burrows believes he is safe at the very center of the earth, in a world that no one knows exists, but the Styx are close behind and will follow him anywhere, while on the surface, Drake and some unlikely allies unearth some critical secrets.

Summary

This book is the fourth book in the Tunnels series. After Will, Elliot and Dr. Burrows jump into "Smoking Jean", Chester and Martha go back to Topsoil, each having adventures of their own. The Styx then come in and start causing trouble.